The invention relates to a device for coating substrates by means of cathode sputtering and comprises
a. a rectangular magnetron cathode which is provided with a target with longitudinal edges and defines a coating region,
b. a substrate holder which can be rotated around a rotating axis that is parallel to the longitudinal edges, the substrates having surface elements which can be moved across the coating region on a circular path,
c. a planar wall which has longitudinal edges and is made of a non-magnetic material, the edges also running parallel to the rotating axis of the substrate holder. A magnet arrangement for the generation of a longitudinally extended magnetic tunnel is disposed behind the wall. This tunnel extends across the surface of wall which faces the substrate. The magnetron cathode and the wall are on opposite sides of the rotating axis. The longitudinal sides of the magnetic tunnel also run parallel to the longitudinal edges of the target, and
d. a planar anode.
The substrates in question are in particular, but not exclusively, those of large dimensions and/or complicated three-dimensional shapes. With conventional devices which have at least one magnetron cathode, it is difficult to coat such substrates uniformly and an all around. Further, it is also difficult to uniformly etch and clean all planar elements prior to the coating. This procedure requires a uniform ion bombardment covering the entire surface.
Using magnetron cathodes, there two requirements which diametrically oppose one another: One the one hand, the magnetron sputtering cathode is characterized by a very particular association between a magnet system and the target which consists of the coating material or at least one component of the coating material, a so called high-performance coating source, i.e. a coating source with a high sputtering rate per surface unit. On the other hand, the plasma and/or the glow discharge is kept away from the substrates by the magnetic field unless other measures of a partly very complex nature are taken. This will be discussed in further detail hereinafter. The interaction between magnetic field and target is also referred to as "magnetic trap."
The properties of a high-performance sputtering process are, as a matter of fact, accomplished by the high concentration of the plasma on the substrate surface. This very fact is the cause for the small spatial expansion of the plasma which opposes the requirement "to bathe" the substrates in the plasma.
Magnetron sputtering cathodes are largely known. For an example, refer to U.S. Pat. No. 4,166,018 which discloses a so-called "planar magnetron."
A magnetron cathode of this kind is not necessarily suitable for the all around coating of three-dimensional substrates, especially, when the substrate is allowed to execute complex composite movements which are made possible by a multi-axial spatial movement of a substrate holder.
From U.S. Pat. No. 4,426,267 it is known to coat three-dimensional substrates all around by passing the substrates across a space formed between two mirror-invertedly opposed magnetron cathodes without a rotational movement. The geometry of the entire arrangement, the design of the magnetron cathode and the operating parameters including a negative bias-voltage at the substrates must be selected such that all sides of the substrate are exposed to the plasma. This measurement proved to be extremely favorable for a substrate size which is prescribed by a distance of the target surfaces of approximately 120 mm. One must take into account that even protruding parts of the substrates have to have a minimum distance from the target surface so that the largest dimension of the substrate in direction of normal of the target surface is limited to a maximum of 30 to 40 mm.
U.S. Pat. No. 4,798,663, discloses apparatus having elements (a.) to (d.) described above. For one particular purpose (coating of so called hard material), it discloses measures and means which permit all sides of three-dimensional substrates to be coated with an adhesive layers of hard material. On the one hand, this is made possible by rotating the substrate, on the other hand, by the bombardment with additional electrons into the coating region of the magnetron cathode. The device is therefore provided with an additional arrangement of electron emitters and counter electrodes which generate an additional flow of electrons. In this arrangement, the substrates are exposed to an additional temperature stress caused by the radiation of the emitters. Temperatures of 200.degree. C. which are harmful to certain materials, for example steel, are already exceeded after a short while. In this piece of prior art, the plasma is spatially very limited. The substrates hence periodically pass through the plasma and plasma-free zones with the largest partial circumference of the substrate paths being outside the plasma zone. In this known device, the maximum size of the substrates also has relatively narrow limits.
With reference to a magnetron cathode, it is also known to increase the spatial expansion of the plasma by operating the magnetron cathode in a condition which can be referred to as unbalanced with respect to the magnet system. Due to differently sized pole surfaces of the magnet systems and/or additional magnetic coils, a substantial portion of the magnetic flux lines does no longer return to the target surface on relatively short arc like paths but it extends far out into the space up to the counter electrode. The latter is usually configured as a substrate holder and holds the substrate. Because of their effect, magnetron cathodes of this kind are also referred to a "Unbalanced Magnetrons." Examples of such systems are described by B. Window and N. Savvides:
1. "Charged particle fluxes from planar magnetron sputtering sources" PA0 2. "Unbalanced" dc magnetrons and sources of high ion fluxes"
J. Vac. Sci. Technol. A 4 (2) March/April 1986, pages 196 to 202 and PA1 J. Vac. Sci. technol. A 4 (3), May/June 1986, pages 453 to 456.
These systems permit a high average substrate current density, however, they fail to provide a sufficiently uniform effect of the plasma on the substrate, particularly during etching and/or cleaning.
In addition to a high density, another important requirement to be met for the successful use of a hard material layer is a sufficient adherence between the substrate and the coating. For this purpose, it is necessary that the substrate material be removed from the entire surface of the component part during the etching or cleaning procedure. In case of a geometrically complicated, large component part, it is crucial that the uniform removal be accomplished solely under the use of a high negative voltage on the substrate. If, however, high negative substrate potentials are applied to the substrate at a pressure of some 10.sup.-2 mbar (argon), only very low current densities of below 0.2 mA/cm.sup.2 substrate surface can be used for the ion bombardment at the substrate during the etching procedure.
The resulting low removal rates lead, on the one hand, to a increased removal of material on the planar elements which are located at the surface and, hence, are easily accessible. On the other hand, there is an excessive etching at the edges. No material or only little material is removed from the covered internal surfaces of the part so that the adherence is already impaired at these locations.
Moreover, it was observed that the relatively high process pressures required for the etching cause a redeposition of removed material from the region of exposed surface parts to those surface parts where there is no or only little etching. Large portions of the substrate surface are thus not subject to any etching.